The present invention relates to the precipitation of nano-particles of zinc sulfide efficiently doped with manganese ions, and their use in a particular electroluminescent device.
ZnS is a well-known phosphor material. It is used in many applications like after-glow phosphors, photon conversion phosphors and electroluminescent phosphors (Cathode Ray Tube displays, Field Emission Displays, Powder Electroluminescent devices, . . . ).
Mn2+-ions in ZnS crystals exhibit a high intense luminescence band at 595 upon excitation of the ZnS host at 330 nm, corresponding to the bandgap of ZnS. This excitation can be done by a photon, resulting in photoluminescence or by an electron, resulting in electroluminescence. In both cases an electron from the 3d5 6s ground state configuration of the Mn2+-ion is excited to a higher level. After non-radiative relaxation to the 3d5 4 g configuration, the electron can decay to the ground state while emitting a photon (luminescence).
From the above it is clear that the more luminescent centres can be incorporated in the phosphor host, the more photons can be emitted for a given amount of host material. The maximum solubility of MnS in ZnS is about 40% at room temperature. This means that on theoretical grounds, a 40% doping level of Mn2+ in ZnS will yield the highest emission. However at these high concentration, concentration quenching of the luminescence occurs, and hence the emission efficiency will drastically decrease. With electron spin resonance techniques it can be nicely shown that the quenching is due to energy transfer from the excited Mn2+ to another neighbouring (non-excited) Mn2+-ion. This means that for an efficient doping the Mn2+-ions should be isolated from each other in the ZnS lattice.
Experimentally and theoretically the maximum doping level of luminescent Mn2+-ions in ZnS is 4%. Above this concentration statistical clustering of Mn2+ will inevitably start to occur, and hence luminescence quenching will result.
ZnS can be doped by Mn2+ by calcinating bulk ZnS and MnS at high temperatures, usually at 700-900xc2x0 C. MnS migrates slowly into the ZnS-lattice. Process optimization (time, temperature, atmosphere, . . . ) leads to highly efficient doped ZnS:Mn phosphors.
Mn2+ can also be incorporated in the ZnS-lattice during coprecipitation of Zn2+ and Mn2+ with S2xe2x88x92-ions. However, it seems to be very difficult to obtain high efficient emitting phosphors with more than 1 at % Mn2+.
Recently(1-8) many reports were published concerning ZnS nano-particles and doped ZnS:Mn nano-particles prepared by using precipitation techniques. Homogeneous co-precipitation of MnS with ZnS is a simple and straightforward technique to incorporate luminescent Mn2+ centres. However, the yield is in most cases low (only about 25% of the added manganese ions will be incorporated in the ZnS lattice). But even worse, concentration quenching starts already at concentrations as low as 1 at %.
In the literature(9-10) addition of methacrylic acid or poly methyl methacrylate during precipitation of ZnS:Mn are described as to enhance the luminescence efficiency. Also addition of 3-methacryloxypropyl trimethoxysilane(11) is ascribed as to enhance the resulting luminescence efficiency.
Many applications for these new class of nano-structured ZnS:Mn phosphors can be thought of as was stated in the introduction.
At present several thin film electroluminescent devices (ELDs) exist or are still in development. They are all characterized by one (or more) electroluminescent active layer(s) sandwiched between two electrodes, of which at least one is transparent. These emitting layers can be organic, inorganic or composite. In analogy with the Oligomeric Light Emitting Diodes (OLEDs) and Polymer Light Emitting Diodes (PLEDs), devices containing inorganic nano-semiconductors will be called hereinafter Inorganic Light Emitting Diodes (ILEDs) or Thin Film Inorganic Light Emitting Diodes.
Recently, several research groups reported electroluminescence(12-16) from inorganic semiconducting nano particles (ILEDs).
Colvin et al.(12) reported on the electroluminescence of CdSe nano-particles stabilized by hexane dithiol. They demonstrated EL for two devices comprising a spincoated double layer of CdSe and PPV (poly(phenylenevinylene)) on ITO and covered it with an evaporated Mg electrode. Depending on the voltage they observed emission from the CdSe (lower voltages) or from the PPV (higher voltages).
Electroluminescence of CdSe quantum-dot/polymer composites was also reported by Dabbousi et al.(13). They spincoated on ITO one single layer of CdSe nano-particles stabilized with trioctylphosphine oxide and mixed with a polymeric hole transporter (PVK) and an electron ransport species (an oxadiazole derivative of PVK, t-Bu-PBD, which is polybenzodiazole). An aluminum electrode was subsequently evaporated. The system showed electroluminescence in reverse bias, and depending on the applied voltage the emission spectrum of the CdSe quantum dots or PVK was observed.
Gao et al.(14) reported on the electroluminescence of self-assembled films of PPV and CdSe nano-particles. They could observe electroluminescence from the CdSe particles and/or from the PPV, depending on the applied voltage.
These examples demonstrate the possible use of inorganic nano-particles with semiconductor properties as Light Emitting Diodes (ILEDs), in analogy with the OLEDs. However, the use of Cd- or Se-compounds can not be recommended due to environmental problems that can be expected.
Huang et al.(15) reported the photo- and electroluminescence of a single layer of ZnS:Cu nanocrystals spincoated on a ITO substrate and evaporated with an aluminum electrode. ZnS and CuxS are much more environmental friendly compared to CdSe. Also there was no need for organic hole or electron transporters, which can cause stability problems as is known in the organic PELDs. The drawback of their system lies in the fact that the synthesis of the ZnS:Cu particles is quite cumbersome and results in low yields. Polystyrene sulphonic acid is used as polyelectrolyte on which Zn and Cu ions are attached. Subsequently this polyelectrolyte is solved in dimethylformamide and reacted with H2S. By this way ZnS:CxS particles are formed.
Que et al.(16) reported photo- and electroluminescence from a copper doped ZnS nanocrystals/polymer composite. The synthesis of the nano-particles was carried out by using the inverse microemulsion method. After washing and drying the ZnS:Cu powder was redispersed in MEK with PMMA as a binder and spincoated on ITO and evaporated with an aluminum electrode. Green electroluminescence could be observed in both bias directions at 5 V. The drawback of the fabrication of this device is the low concentrations of the ZnS:Cu dispersion that can be obtained (ca 10xe2x88x923 M). Further it needs a well defined two phase system (soap/water). Also a drawback for future industrial application could be the solvent based spincoating dispersion.
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It is the object of the present invention to provide a process for the preparation of a nanoparticle dispersion of zinc sulfide doped in a very efficient way with manganese ions (ZnS:Mn).
It is a further object of the present invention to provide such a process wich is easy and economical from the manufacturing point of view.
It is still a further object of the present invention to provide a Thin Film Inorganic Light Emitting Diode comprising a coated layer containing such an improved ZnS:Mn dispersion.
The objects of the present invention are realized by providing a process for the preparation of a nanoparticle dispersion of zinc sulfide doped with manganese ions (ZnS:Mn) comprising the step of performing a precipitation by mixing appropriate aqueous solutions of zinc cations, sulfide anions and manganese cations, characterized in that one or more of said aqueous solutions further contains a triazole or diazole compound.
A Thin film Inorganic Light Emitting Diode comprising a coated layer containing such a ZnS:Mn dispersion also belongs to the scope of the present invention.